In addition to an increase in optical communication capacity, the miniaturization and the low electric-power consumption of optical devices used therein have been demanded. Mach-Zehnder (MZ) optical modulators are exemplified as important element devices that involve in those characteristics, and many researches and developments thereof have been promoted. Particularly in recent years, in order to achieve the further miniaturization and lower electric-power consumption, an attention is specially focused on optical modulators using compound semiconductor materials including InP from optical modulators using lithium niobate (LN) as a material.
The characteristics of compound semiconductor optical modulators are briefly described below. In order to cause the compound semiconductor optical modulator to operate as an optical modulator, the interaction between electricity (electric field) and propagation light is utilized. The strong interaction between light and electricity is achieved by using a core layer that generally traps the light as a non-doped layer, sandwiching the core layer between p-type and n-type clad layers in the order from the top layer, and applying a reverse bias voltage thereto. Meanwhile, the p-type semiconductor used herein has a higher electrical resistivity and a higher optical absorption of material, than the n-type semiconductor, by approximately one or more order of magnitude, so that there are large problems in achieving higher-speed modulation operation and lower optical loss. In addition, a contact area between the p-type semiconductor and an electrode is sufficiently small compared with that of the n-type, so that the increase in contact resistance causes further degradation of modulation bandwidth. Approaches by the following two main ways (the improvement in the semiconductor layer and the improvement in the electrode structure) have been made in order to solve these problems.
As the approach by the improvement in the semiconductor layer, a semiconductor optical modulation element having a cross-sectional structure as illustrated in FIG. 1 was proposed in the past (for example, Patent Literatures 1 and 2). The semiconductor optical modulation element illustrated in FIG. 1 has a waveguide structure of a deep-ridge or high-mesa shape that is stacked on a substrate 10. Although the high-mesa waveguide is used in FIG. 1, this is a configuration generally employed in a semiconductor MZ modulator for strengthening the interaction between light and electricity and reducing the parasitic capacitance. A signal electrode 16 is provided on the waveguide structure via an n-type contact layer 15. A ground electrode 17 is provided on an n-type clad layer 11. FIG. 1 illustrates an npin type optical modulator in which clad layers 11 and 14 above and below a non-doped layer 12 are served as the n-type, a thin p-type layer serving as a carrier block layer 13 for suppressing the electron current between the both n layers is inserted. This npin type does not use a p-type clad that has large optical loss, thereby making it possible to use a relatively long waveguide. Moreover, the npin type is easy to simultaneously satisfy the reduction in drive voltage and the velocity match between a microwave and light wave because of the flexibility in which the thickness of a depletion layer can be appropriately designed as desired, and thus has an advantage to increase the response speed of the modulator. The RF responses calculated by calculation simulation in the npin layer structure is illustrated in FIG. 2.
However, the npin type modulator having a high-mesa shape as is in FIG. 1 involves the large variation in electric surge resistance and the instability of modulation operation, as problems. The main factor of the variation in surge resistance is considered to be in that the damage along with the waveguide side walls caused by dry etching processing lowers the function as a carrier block layer of the p-type semiconductor. The instability of modulation operation has specifically problems including generation of the incident light power dependence of the modulation efficiency, generation of the bias voltage dependence of the modulation efficiency, and the lowering of the resisting pressure. It can be considered that the main factor of this is in that hole carriers are concentrated and accumulated on the p layer in the high-mesa waveguide. In other words, hole carriers are concentrated and accumulated on the p layer in the high-mesa waveguide to cause such concerns as generation of the incident light power dependence and the bias voltage dependence in modulation efficiency, lowering of the resisting pressure, and the like. Although various proposals with respect to these problems have been made (Patent Literatures 3 and 4), all of these proposals require complicated structures and the optimization of the material resistance, so that many problems as a practical device thus remain. As a result, in the current research and development, the most of research institutes employ the conventional pin structure from the viewpoint of the electrical stability (Non Patent Literature 1).
In addition, electroabsorption (EA) type modulators of a nip structure in which an upper layer clad is of n-type, a lower layer clad and a substrate are of p-type for reduction in contact resistance have been proposed (Patent Literature 5). However, the EA modulator generally has an electrode structure of a lumped constant type, and thus does not need to satisfy the characteristic impedance, the velocity matching between a light wave and microwave, and the like. Meanwhile, in the MZ modulator having an electrode length longer by approximately one order of magnitude than that of the EA modulator, it cannot be said that simply reversing the semiconductor layer structure is not sufficient for the improvement in the modulation bandwidth. This is apparent because only about 5 GHz of the bandwidth improvement is expected from the comparison between p-i-n and n-i-p in FIG. 2. Moreover, as in Patent Literature 4, using a conductive substrate (p-type substrate) results in a lager parasitic capacitance compared with that of a semi-insulating substrate, and therefore employing this technique without any change to an MZ modulator using the traveling-wave type electrode structure still has problems.
As an approach by the improvement in the semiconductor layer, a cross-sectional structure as illustrated in FIG. 3 has been proposed (for example, Non Patent Literature 1). A capacitance-loaded electrode structure is employed in FIG. 3, in which a coplanar-strip line including electrodes with large cross-sectional areas is used as a main line, and a lumped constant electrode (capacitance) which is separated in a length sufficiently shorter than the micro wave is added on an optical waveguide. A signal electrode 16 and a ground electrode 17 are provided on a p-type clad layer 19 via a p-type contact layer 20. This allows the high-speed operation thanks to the low loss of the main line to be expected, and also allows the characteristic impedance and the microwave velocity to be independently controlled, thus having extreme high design flexibility.